Fix the build

There are no binaries for 1.8.7 or rbx-3 in Travis CI, so drop them.

For Ruby 2.1+ Travis CI installs the right version of bundler.
If bundler is not installed, install bundler 1.x, since that's
the version that supports those old Ruby versions.

.travis.yml

@@ -1,25 +1,23 @@
 sudo: false
 
-before_install: gem install bundler
+before_install:
+  - which bundler || gem install bundler -v '< 2'
 
 rvm:
-  - 1.8.7-head
   - 1.9.3-p551
   - 2.0.0-p648
   - 2.1.10
   - 2.2.9
   - 2.3.6
   - 2.4.3
   - 2.5.0
-  - rbx-3
   - jruby-19mode
   - ruby-head
   - jruby-head
 
 matrix:
   fast_finish: true
   allow_failures:
-    - rvm: rbx-3
     - rvm: jruby-19mode
     - rvm: ruby-head
     - rvm: jruby-head