hlttracking parabolic magnetic field in building and fitting

I added the missing config file update 
MagneticField_38T_PostLS1_cff.py
and now the error spotted in https://github.com/cms-sw/cmssw/pull/2244#issuecomment-33709896
seems to be fixed

1306.0_SingleMuPt1_UP15+SingleMuPt1_UP15+DIGIUP15+RECOUP15+HARVESTUP15 Step0-PASSED Step1-PASSED Step2-PASSED Step3-PASSED  - time date Tue Feb  4 15:57:32 2014-date Tue Feb  4 15:45:01 2014; exit: 0 0 0 0
1 1 1 1 tests passed, 0 0 0 0 failed

@cerati @rovere @Martin-Grunewald @perrotta