Remove PGI Experimental Support

Can not reproduce stable builds any more - skip support for now until
there is basic host side support from NVidia (post-6.0).
GCC 4.7 and Boost 1.49+

- (doc) CUDA 5.5 supports GCC 4.7 (and we do, too)
- (fix) we require boost 1.49 for a long time now, due to "move"
  that was introduced in it (see CMakeLists.txt)
- (fix) add sudo
Add precise information about driver

CUDA 5.0 introduced a driver bug (mainly ECC false negative
errors) between the 5.0 early access/release candidate and the
community release. Due to that, we simply recommend to update
to the latest 5.5 or soon to be 6.0 drivers.

Remember: you can always run an *older* cuda toolkit with a
newer driver (but not vice versa).
Merge pull request #257 from ax3l/doc-noPGI

Remove PGI Experimental Support