Fix build on Travis

Use require_relative for capybara_webkit_builder as RVM
on travis doesn't include lib directory in require paths
Skip forking spec on JRuby

* JRuby doesn't support `fork`, so this test is irrelevant.
Read the expected number of bytes from StringIO

* Instead of sleep, use blocking #read to determine if the expected
  message is logged.
* Fixes failure 2 from #615.
Use a pipe to test command logging

* JRuby doesn't write to StringIO in the same way as cruby/rbx, and the
  #read method used in connection_spec didn't work here.
* Fixes failure 3 from #615.
Build against JRuby (1.9 mode)
Use a pipe to test stderr forwarding

* Assertion was accidentally removed when testing #read as an
  alternative.
Add JRuby 2.0 to travis build