update tarpaulin use to accomidate xd009642/tarpaulin#150

.travis.yml

@@ -23,8 +23,10 @@ addons:
       - libssl-dev
 
 install: |
-  if [[ "$TRAVIS_RUST_VERSION" == stable ]] && [[ $(cargo tarpaulin --version) != *0.6.2 ]]; then
-    `RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install  --force --version 0.6.2  cargo-tarpaulin`
+  # https://github.com/xd009642/tarpaulin/issues/150
+  rustup toolchain install nightly
+  if [[ "$TRAVIS_RUST_VERSION" == stable ]] && [[ $(cargo +nightly tarpaulin --version) != *0.6.2 ]]; then
+    `RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo +nightly install  --force --version 0.6.2  cargo-tarpaulin`
   fi
 
 script:
@@ -35,7 +37,7 @@ script:
 after_success:
   # report coverage to coveralls
   # see https://github.com/xd009642/tarpaulin for more information
-  - cargo tarpaulin --ciserver travis-ci --coveralls $TRAVIS_JOB_ID || true
+  - cargo  +nightly tarpaulin --ciserver travis-ci --coveralls $TRAVIS_JOB_ID || true
 
 # deploy on pushes to master branch
 # assumes aws credentials (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY)