Align scala-reflect version with Scala version in the project.

This way spire build supports cross-versioning against multiple versions of
Scala properly.
Add dependency on macro paradise only when Scala 2.10 is used

Spire uses macro paradise plugin just for quasiquotes. Quasiquotes got
merged into Scal reflection in Scala 2.11 so there's no need to depend on
the macro paradise plugin in case Scala 2.11 is used.

This commit makes inclusion of the plugin conditional on the Scala version
used in the build.
Get rid of reference to ambiguously imported `sqrt`.

In Complex.scala, the `sqrt` identifier is brought to scope by two
different means:

  * the `math` package object declared `sqrt` def
  * it's imported explicitly from java.lang.Math.sqrt

Scala 2.11 started to (properly) flag this ambiguity so we need to resolve
it explicitly.
Add Scala reflection source compatibility hack.

Scala reflection has been heavily refactored in Scala 2.11 but a source
compatibility hack has been introduced. We make use of it in spire's
macros.