Fixed base64 decode parameter for newer versions of base64 binary

In OS X, the shipping binary for "base64" uses -D has the decode parameter. This is not supported in newer versions of base64 so people that have installed Homebrew's "coreutils" brew, or even on non-OS X operating systems will get:

    % decode64 d2VsY29tZQ==
    base64: invalid option -- 'D'
    Try 'base64 --help' for more information.

This commit will modify the decode parameter to use "--decode" which works across all versions of base64.