Update strings.sol according to latest compiler version

[chriseth]

No description provided.

[danielscai]

my latest complier is solidity 0.4.13 , and i change pragma solidiy ^0.4.13 .

and i got the error

 Jump instructions are low-level EVM features that can lead to incorrect stack access. Because of that they are discouraged. Please consider using "switch" or "for" statements instead.
                    jumpi(exit, eq(and(mload(ptr), mask), needledata))
                    ^---^
, 

[chriseth]

@danielscai yes, this does not yet remove all warnings, but I would say it is an improvement.

[Arachnid]

Thanks!