generateCert function's sed command issue for osx.

[bosoa]

ISSUE: When running ./trade.sh generate -c tradechannel command for testing, I found sed command has issue on osx.

SOLVE: change [sed -i] with [sed -i ""].

REASON: Unlike Ubuntu, OS X requires the extension to be explicitly specified. The workaround is to set an empty string:
$ sed -i '' 's/megatron/pony/g' /path/to/file.txt
(ref: lmquang/til#18)

[VRamakrishna]

Fixed with da0d715