Solidity version fix.
Solidity version fix.

This contract will only compile with 0.4.16 and higher versions. 

0.4.15 and earlier compiler will throw the following error because of `pure` keyword:

```
browser/NettingChannelLibrary.sol:389:9: ParserError: Expected token LBrace got reserved keyword 'Pure'
        pure
        ^
```
hackaugusto/raiden,hackaugusto/raiden