Update PHP vesion requirements

Fixes #15

Merge pull request #14 from loostro/bugfix-value-range-less-than-input

Fix #13 Added expection when input value is out of range

Merge pull request #12 from loostro/bugfix-error-in-comparison-11

Fixed gmp_cmp return value comparison

Merge pull request #9 from loostro/feature-more-cipher-methods

Add more cipher methods