Recommend caret constraints in "Installing" tab

[seaneagan]

Currently it recommends e.g.:

dependencies:
  prompt: ">=0.1.3 <0.2.0"

It would be nice to recommend caret constraints instead:

dependencies:
  prompt: "^0.1.3"

Maybe could recommend both until enough folks have upgraded to dart 1.8.0 when ^ was introduced.

[zoechi]

Maybe suggest adding

environment:
  sdk: ">=1.8.0"

to the second variant

[seaneagan]

@zoechi yep