removes -no-share from sbt invocations in macro builds

I found that this sometimes leads to spurious failures like, which
disappear when I remove -no-share.

sbt.InvalidScalaInstance: Scala instance doesn't exist or is invalid:
    version 2.10.4, library jar: /localhome/jenkins/c/workspace/macro-paradise-nightly-2.10.x/job/macro-paradise-210x/paradise/project/.ivy/cache/org.scala-lang/scala-library/jars/scala-library-2.10.4.jar, compiler jar: /localhome/jenkins/c/workspace/macro-paradise-nightly-2.10.x/job/macro-paradise-210x/paradise/project/.ivy/cache/org.scala-lang/scala-compiler/jars/scala-compiler-2.10.4.jar
(E.g. see https://scala-webapps.epfl.ch/jenkins/view/misc/job/macro-paradise-nightly-2.10.x/474/console)

The errors started appearing when I committed
https://github.com/scalamacros/paradise/commit/d1284c776cac657b91a2e63f59bdbd8fd7dba50b,
where I set Scala version to 2.10.4 in a subproject of a big project
that uses Scala version 2.10.x.
Merge pull request #129 from xeno-by/master

removes -no-share from sbt invocations in macro builds