Add `rvm install 2.3.3` to .travis.yml (#151)

.travis.yml

@@ -69,6 +69,7 @@ before_install:
   - |
     wget "https://raw.githubusercontent.com/boostorg/release-tools/develop/ci_boost_common.py" -P ..
     wget "https://raw.githubusercontent.com/boostorg/release-tools/develop/${SCRIPT}.py" -P ..
+    rvm install 2.3.3
 
 install: python "${TRAVIS_BUILD_DIR}/../${SCRIPT}.py" install
 before_script: python "${TRAVIS_BUILD_DIR}/../${SCRIPT}.py" before_script