remove jruby-18mode from travis

```
$ rvm use jruby-d18 --install --binary --fuzzy
jruby-d18 is not installed - installing.
Searching for binary rubies, this might take some time.
Requested binary installation but no rubies are available to download, consider skipping --binary flag.
Gemset '' does not exist, 'rvm jruby-d18 do rvm gemset create ' first, or append '--create'.
The command "rvm use jruby-d18 --install --binary --fuzzy" failed and exited with 2 during .
```

.travis.yml

@@ -27,8 +27,6 @@ matrix:
     gemfile: gemfiles/Gemfile-edge
   - rvm: 2.3.0
     gemfile: Gemfile
-  - rvm: jruby-18mode
-    gemfile: gemfiles/Gemfile-ruby-1.8.7
   - rvm: jruby-19mode
     gemfile: gemfiles/Gemfile-ruby-1.9
   - rvm: jruby-head