Clean build.sbt a little

First off, it seems that lines https://github.com/typelevel/wartremover/blob/d6b95c3a3720be45aff2d4093256671dd2ebf8ba/build.sbt#L54-L59 are not used anymore.

Fixed imports to be more explicit (previous were `red code' in IntelliJ).

Also changed scalac version to 2.11.1 (2.11.0 should not be used, although its bug does not touch WR) and updated scm links in `pomExtra`. =)
Fix xmlSymbols in Null wart for scalac 2.11

Before, when a project using scala 2.11 did not use:

```scala
"org.scala-lang.modules" %% "scala-xml" % whatever
```

an exception would be thrown at the beginning of its compilation:

```
[error] (root/compile:compile) scala.ScalaReflectionException: object scala.xml.Elem in compiler mirror not found.
```
A wart for detecting places where using a Monad instance is redundant,

and an Applicative instance will do.
Add description of NoNeedForMonad to README.
Merge pull request #87 from folone/monads-applicatives

A wart for detecting places where using a Monad instance is redundant
Merge pull request #90 from michalrus/fix-xml-2.11

Fix xmlSymbols in Null wart for scalac 2.11
Merge remote-tracking branch 'michalrus/clean-build-sbt'

Conflicts:
	build.sbt
Add license info to README