Merge pull request #155 from joneslee85/patch-1

Tell Travis to test against the current branch's latest commit

.travis.yml

 language: c
-script: asdf plugin-test ruby https://github.com/asdf-vm/asdf-ruby.git
+script: asdf plugin test ruby . --asdf-plugin-gitref $TRAVIS_BRANCH ruby -v
 before_script:
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then export TRUFFLERUBY_RECOMPILE_OPENSSL=false; fi
   - git clone https://github.com/asdf-vm/asdf.git