Add workaround for rubygems/rubygems#763

This defect in Rubygems 2.2.0 breaks all 1.8.x builds on Travis CI.

Workaround until the fix is deployed is to pin builds to latest 2.1.x
build of Rubygems.

.travis.yml

@@ -1,4 +1,8 @@
 language: ruby
+# NOTE: temporary workaround for Rubygems issue #763
+before_install:
+  - gem update --system 2.1.11
+  - gem --version
 rvm:
   - 2.1.0
   - 2.0.0