Allow ruby pessimistic operator (~>) as well as tilde (~)

[patcon]

It would be great if ~ and ~> were parsed as the same operator in composer. This would make it simpler for rubyists to walk in composer's world, without sacrificing anything for those already using composer.

Came across the below issue, which makes me think I'm understanding our current behaviour correctly, and that the suggestion would make sense:
#1679

Here's an external explanation of the pessimistic operator, proving that they're the same thing in ruby-land.
http://robots.thoughtbot.com/rubys-pessimistic-operator

[patcon]

https://npmjs.org/doc/misc/semver.html

Hm. Seems npm's semver operator ~ works the same way as ruby's. Not sure why we don't all make our operators cross-compatible. I guess that's my suggestion then :)

[Seldaek]

First of all we can't really change this without breaking BC. That said, I don't think it needs changing. We do what bundler's ~> does, and we are close to what npm's ~ does, except where it disagrees with bundler.

constraint	composer	bundler	npm	summary
~1.2.3	>=1.2.3 && < 1.3.0	>=1.2.3 && < 1.3.0	>= 1.2.3 && < 1.3.0	all equal
~1.2	>=1.2.0 && < 2.0	>=1.2.0 && < 2.0	>=1.2.0 && < 1.3.0	npm differs

As I pointed out in that other issue, npm's ~1.2 behavior is equivalent to 1.2.* so I don't think it's particularly superior.

As for supporting ~ or ~>, I guess we could "support it" with a warning just so people mistyping it get nudged in the right direction. I don't think having two operators doing the same is a good thing.