Pass tests on 0.7

[mlubin]

• No longer allow failures on 0.7 on CI
• Fix any failing tests until we pass:

Test Summary: | Pass  Total
MOI           |   38     38
Quadratic: Error During Test at /home/mlubin/.julia/dev/MathOptInterface/test/functions.jl:139
  Got exception MethodError(zero, (nothing,), 0x0000000000006c68) outside of a @test
  MethodError: no method matching zero(::Nothing)
  Closest candidates are:
    zero(!Matched::Type{LibGit2.GitHash}) at /home/mlubin/julia/julia-0.7/usr/share/julia/stdlib/v0.7/LibGit2/src/oid.jl:210
    zero(!Matched::Type{OldPkg.Resolve.VersionWeights.VWPreBuildItem}) at /home/mlubin/julia/julia-0.7/usr/share/julia/stdlib/v0.7/OldPkg/src/resolve/versionweight.jl:80
    zero(!Matched::Type{OldPkg.Resolve.VersionWeights.VWPreBuild}) at /home/mlubin/julia/julia-0.7/usr/share/julia/stdlib/v0.7/OldPkg/src/resolve/versionweight.jl:116
    ...
  Stacktrace:
   [1] iszero(::Nothing) at ./number.jl:32
   [2] _modifycoefficient(::Array{MathOptInterface.ScalarAffineTerm{Int64},1}, ::MathOptInterface.VariableIndex, ::Int64) at /home/mlubin/.julia/dev/MathOptInterface/src/Utilities/functions.jl:313
   [3] modifyfunction(::MathOptInterface.ScalarQuadraticFunction{Int64}, ::MathOptInterface.ScalarCoefficientChange{Int64}) at /home/mlubin/.julia/dev/MathOptInterface/src/Utilities/functions.jl:332
   [4] macro expansion at /home/mlubin/.julia/dev/MathOptInterface/test/functions.jl:143 [inlined]
   [5] macro expansion at /home/mlubin/julia/julia-0.7/usr/share/julia/stdlib/v0.7/Test/src/Test.jl:1079 [inlined]
   [6] macro expansion at /home/mlubin/.julia/dev/MathOptInterface/test/functions.jl:140 [inlined]
   [7] macro expansion at /home/mlubin/julia/julia-0.7/usr/share/julia/stdlib/v0.7/Test/src/Test.jl:1079 [inlined]
   [8] macro expansion at /home/mlubin/.julia/dev/MathOptInterface/test/functions.jl:118 [inlined]
   [9] macro expansion at /home/mlubin/julia/julia-0.7/usr/share/julia/stdlib/v0.7/Test/src/Test.jl:1079 [inlined]
   [10] top-level scope at /home/mlubin/.julia/dev/MathOptInterface/test/functions.jl:2
   [11] include at ./boot.jl:314 [inlined]
   [12] include_relative(::Module, ::String) at ./loading.jl:1071
   [13] include(::Module, ::String) at ./sysimg.jl:29
   [14] include(::String) at ./client.jl:393
   [15] macro expansion at /home/mlubin/.julia/dev/MathOptInterface/test/runtests.jl:35 [inlined]
   [16] macro expansion at /home/mlubin/julia/julia-0.7/usr/share/julia/stdlib/v0.7/Test/src/Test.jl:1079 [inlined]
   [17] top-level scope at /home/mlubin/.julia/dev/MathOptInterface/test/runtests.jl:35
   [18] include at ./boot.jl:314 [inlined]
   [19] include_relative(::Module, ::String) at ./loading.jl:1071
   [20] include(::Module, ::String) at ./sysimg.jl:29
   [21] include(::String) at ./client.jl:393
   [22] top-level scope
   [23] eval(::Module, ::Any) at ./boot.jl:316
   [24] exec_options(::Base.JLOptions) at ./client.jl:244
   [25] _start() at ./client.jl:427
linear1: Error During Test at /home/mlubin/.julia/dev/MathOptInterface/src/Test/config.jl:42

• Longer term, we should set --depwarn=error in the CI.

[blegat]

coalesce needs to be replaced by something. something has been added to Compat 3 days ago: JuliaLang/Compat.jl#562 and no release contain this change yet.