Add support for JRuby-1.7.{5,6}

The method RubyString.encode_bang(ThreadContext) was removed, so call
the array version with an empty array instead.
Disable a lot of travis tests

Don't run 1.8-mode as it does not work with "encode!"
Don't run jruby 1.6.8 as bundler gets stuck in an endless redirect loop
Version bump to v1.4.0.pre2