Fix compilation for Qt4 and remove QT_VERSION_4 define

- QStringLiteral used in qofonoconnectioncontext.h is Qt5 only
  check Qt version before using it

- Remove the QT_VERSION_4 define use the QT_VERSION macro instead.

Signed-off-by: Ivan Pintado <vpintado@cercacor.com>
Merge pull request #25 from ivanph/master

Fix compilation for Qt4 and remove QT_VERSION_4 define