Only build on MRI 1.9.3 for now

We'll have to get a handle on these weird build failures, some of which
seem to be flickering. But let's take it one step it at a time and focus
on getting it running reliably on at least *one* Ruby version.

.travis.yml

@@ -1,31 +1,5 @@
 rvm:
-  - 1.8.7
-  - 1.9.2
   - 1.9.3
-  - ree
-  - jruby-18mode
-  - jruby-19mode
-  - rbx-18mode
-  - rbx-19mode
-gemfile:
-  - Gemfile
-  - gemfiles/Gemfile.base-versions
-matrix:
-  exclude:
-    # Nokogiri 1.3.3 is not compatible with Rubinius or JRuby
-    - gemfile: gemfiles/Gemfile.base-versions
-      rvm: rbx-18mode
-    - gemfile: gemfiles/Gemfile.base-versions
-      rvm: rbx-19mode
-    - gemfile: gemfiles/Gemfile.base-versions
-      rvm: jruby-18mode
-    - gemfile: gemfiles/Gemfile.base-versions
-      rvm: jruby-19mode
-  allow_failures:
-    - gemfile: Gemfile
-      rvm: rbx-19mode
-    - gemfile: Gemfile
-      rvm: jruby-19mode
 before_script:
   - sh -e /etc/init.d/xvfb start
   - export DISPLAY=:99.0