Silence a compiler warning

The compiler doesn't know that `rb_raise` longjmps us out of the
function, so make it feel better by pretending that we'll return a VALUE
no matter what.

Fixes the Clang warning:

  watchman.c:470:1: warning: control may reach end of non-void function [-Wreturn-type]

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Fix compilation errors on 1.8

1.8 has an st.h header which defines ST_CONTINUE (et al), which is
apparently not included by default.

1.9 has a ruby/st.h header, included by default, and also a st.h header
for backwards compatibility. The latter works but issues a warning to
the console.

Let's check for the presence of the headers and use the appropriate one.
Note that ruby/st.h has the standard include guards in place, so we need
not worry about inadvertently including things twice.

Reported here:

  https://github.com/wincent/Command-T/issues/67#issuecomment-38815959

Leads to:

  watchman.c:188:12: error: use of undeclared identifier 'ST_CONTINUE'

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Update specs for Ruby 1.8 compatibility

The implementation does work under 1.8, but the specs needed some
convincing of that fact.

Signed-off-by: Wincent Colaiuta <win@wincent.com>