Error Use of const in strict mode.

[mar10]

Not sure if this is this a regression, but this test fails
on node v0.12.18

>> SyntaxError: /home/travis/build/axemclion/grunt-saucelabs/node_modules/requestretry/strategies/HTTPError.js:9
>>   const statusCode = response ? response.statusCode : null;
>>   ^^^^^
>> Use of const in strict mode.

(See also axemclion/grunt-saucelabs#233)

I assume this was introduced here #85 with this commit: 475ea26

[FGRibreau]

(Closing too old)