Fix build issues (see details)

Builds on Travis were failing, returning an error "Could not find 'bundler' (1.16.2) required by your Gemfile.lock".  It seems this problem was caused by a forced upgrade of Ruby Gems, which, in turn, was caused because our Travis RVM config forces an old version of Ruby (2.3.3) which is not supported by the Travis build environment (see https://travis-ci.org/sul-cidr/noh/builds/629005327, esp. the `rvm use 2.3.3 --install --binary --fuzzy` command at line 176 and the `ruby --version` command at line 207).

The CI environment has Ruby 2.3.8 pre-installed, and switching the RVM config to use this version enables the use of the pre-installed `bundle` executable.  This allows builds to complete, and has the pleasant side-effect of speeding up the build-time considerably, too (see https://travis-ci.org/sul-cidr/noh/builds/629162946 -- I ran a manual build on Travis with a custom config).

It seems that RVM has no `2.3.x` syntax, or equivalent (see rvm/rvm#3512), which is disappointing.  However, it seems Travis does have functionality whereby specifying a `MAJOR.MINOR` without a `.PATCH` causes the build environment to use its pre-installed binaries for that version (see travis-ci/travis-ci#7077 and https://docs.travis-ci.com/user/languages/ruby/#specifying-ruby-versions-and-implementations).  This has the advantage of avoiding the problem encountered today, when Travis updates its build images, but presumably means that builds will fail if/when Ruby 2.3 is removed from those images.  On balance, this seems the lesser evil, so this commit sets the Travis RVM config to `2.3`.

Committing directly to `contents` in the interest of expediency.

.travis.yml

@@ -1,7 +1,7 @@
 language: ruby
 
 rvm:
-  - 2.3.3
+  - 2.3
 
 install:
   - nvm install v8.4