Fix time argument parsing for gencrl

[Xeroday]

Usage of gencrl:
cfssl gencrl INPUTFILE CERT KEY TIME

Arguments:
...
TIME (OPTIONAL): The desired expiration from now, in seconds

This PR uses the time argument as a string (as expected from the usage). The current code takes the time arg and uses it as a file name to read from.

Eg:
PR: ./cfssl gencrl serials.txt cert.pem key.pem 86400
Current: ./cfssl gencrl serials.txt cert.pem key.pem time.txt where time.txt has contents '86400'.

[codecov-io]

Current coverage is 58.29% (diff: 100%)

Merging #683 into master will increase coverage by 0.08%

@@             master       #683   diff @@
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  Files            74         74          
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  Methods           0          0          
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  Branches          0          0          
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+ Hits           3746       3750     +4   
+ Misses         2330       2323     -7   
  Partials        360        360          

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