Set CYLC_VERSION then source .profile, for cylc access.

Used at the top of job scripts to give tasks access to cylc, and as the
first part of event handler and job poll and kill command strings so
that they also run under the same version of cylc as the calling suite
(event handlers may call cylc commands). This allows users to run suites
under multiple cylc versions at once by setting PATH according to the
cylc version.

Addresses #471