[BUGFIX] Improve resolution of core migration ``version`` argument
[BUGFIX] Improve resolution of core migration ``version`` argument

By specifying the ``version`` flag on the ``./flow core:migrate``
command a specific core migration can be targeted.

This change improves the regular expression that extracts the numeric
identifier from that flag:
Previously when specifying the whole version string as in
``TYPO3.Fluid-20150214130800`` the resulting version number would
include the "3" of the package key.

Change-Id: Id23b312c7dae09bb5be9d84449775b1e4bf9e2c5
Related: FLOW-179
Releases: master, 3.0
Original-Commit-Hash: 7cd083e0499ae02f159644827afeddc731aecfa3

mkeitsch/flow-development-collection,dfeyer/flow-development-collection,albe/flow-development-collection,dlubitz/flow-development-collection,kitsunet/flow-development-collection,dlubitz/flow-development-collection,gerhard-boden/flow-development-collection,robertlemke/flow-development-collection,aertmann/flow-development-collection,fcool/flow-development-collection,robertlemke/flow-development-collection,daniellienert/flow-development-collection,neos/flow-development-collection,kitsunet/flow-development-collection,albe/flow-development-collection,gerhard-boden/flow-development-collection,robertlemke/flow-development-collection,neos/flow-development-collection,bwaidelich/flow-development-collection,fcool/flow-development-collection,aertmann/flow-development-collection,dfeyer/flow-development-collection,aertmann/flow-development-collection,fcool/flow-development-collection,dfeyer/flow-development-collection,dlubitz/flow-development-collection,gerhard-boden/flow-development-collection,bwaidelich/flow-development-collection,daniellienert/flow-development-collection,fcool/flow-development-collection,albe/flow-development-collection,robertlemke/flow-development-collection,neos/flow-development-collection,kdambekalns/flow-development-collection,chewbakartik/flow-development-collection,daniellienert/flow-development-collection,bwaidelich/flow-development-collection,kitsunet/flow-development-collection,neos/flow-development-collection,kdambekalns/flow-development-collection,mkeitsch/flow-development-collection,chewbakartik/flow-development-collection,kdambekalns/flow-development-collection,kdambekalns/flow-development-collection,mkeitsch/flow-development-collection,chewbakartik/flow-development-collection,dfeyer/flow-development-collection