tab: remove #reliable_runtime_dependencies?

See https://github.com/Homebrew/brew/pull/1750#discussion_r94243825 for
discussion.

Removes Tab#reliable_runtime_dependencies? in favour of returning nil
from Tab#runtime_dependencies if the list is unreliable.

Because Homebrew 1.1.6 hasn't been tagged yet, tabs created in tests
aren't created with a homebrew_version that marks the
runtime_dependencies in the Tab as reliable, so there are some tests
that fail. To work around this, I've had to add a line to some tests
that explicitly overrides the homebrew_version in the Tab. This is
really ugly though, so they should be removed as soon as possible after
1.1.6 is released.