fix ArrayJavaProxy#to_a failure on arrays containing null

https://github.com/jruby/jruby/commit/b96b2219f9df6622883a88eda0ae1dbbcb5c2267 added an unprotected method call in a place where the
target could be null. This adds a null check to that expression.
Merge pull request #1809 from ratnikov/loadservice-enebo-comments

apply minor enebo's comments
apply enebo's comments on loadService refactor.
Merge pull request #1780 from tduehr/module_prepend

Module prepend
Merge pull request #1803 from cprice404/fix-set-native-enabled

Fix minor bug in `setNativeEnabled`, and expose it in ScriptingContainer
Merge pull request #1805 from grddev/zlib-crc-improvements

Zlib crc improvements
Merge pull request #1540 from jruby/vm-version

Include vm.version in --version string
Merge branch 'jruby-1_7' of jruby.org:jruby into jruby-1_7
IT to show how many jars with embedded gems can be used right away without any "extra" configuration
follow the version changes from pom.xml [skip ci]
Implement Module#singleton_class? predicate method

This method has been introduced in Ruby 2.1 and is a convenient way to
check whether a class is a singleton one or not (see
https://bugs.ruby-lang.org/issues/7609 for further information).
cleanup maven snapshot repo on ci.jruby.org [skip ci]
allow openssl to work even without native ffi support - fixes #1800
add missing ITs for the various maven artifacts
Merge branch 'jruby-1_7' of github.com:jruby/jruby into jruby-1_7
some updates on how to run tests
[skip ci]
remove by now obsolete jruby-rake-plugin - not part of build since quite some time
[skip ci]
* Update the link on testing in the README.md.
* Update the test command to make sure the right jruby and rake binaries are used.
fix typo [skip ci]