Fixed `-@` unary op. precedence

With a numeric and followed by a method call was
previously interpreting `-1.foo` as `-(1.foo)`.

fixes #554
Use thin as Webrick has some problems with Safari
Update Guardfile, now should take less cpu
Remove some MRI warnings
Show more error info in $DEBUG/$VERBOSE mode
No “-@” method call when followed by num literal

The same is true for “+@”. ref: #554