Move ava as devDependencie

[YPetremann]

fix #9

[broofa]

The postcss dependency should be moved to peerDependencies as well, and given a suitable semver modifier. Also, the README suggests using autoprefixer, so that should probably get added as an optional peer dependency. I.e. the dependencies should look more like this:

"dependencies": {},
"devDependencies": {
    "ava": "^0.20.0",
},
"peerDependencies": { 
  "postcss": ">5.0.1",
  "autoprefixer": "*"
},
"peerDependenciesMeta": {
  "autoprefixer": {
    "optional": true
  }
}

Note: Given how minimal the unit tests are, I'd suggest just removing the ava dependency altogether and instead use Node's built-in test runner.

... but I wouldn't do any of that until @archana-s indicates she is interested in taking PRs and publishing new versions of this module. (It's unclear if this project is still on her radar.)