Update PerfUtil to latest version to avoid binding exceptions

[mausch]

I'm getting compile-time errors while upgrading from the currently targeted version 0.1.5 to 0.1.8:

• No assignment given for field 'Repeat' of type 'PerfUtil.PerfTest<_>' Fuchu.PerfUtil\PerfUtil.fs 19
• No overloads match for method 'PastImplementationComparer```1'. Fuchu.PerfUtil\PerfUtil.fs 114

Please send me a pull request or point me to whatever I have to change about this.

[haf]

see https://github.com/mausch/Fuchu/pull/38/files?w=1#diff-d37fa8a24fbfee6d8485ba58613659ccR19

[mausch]

Cheers, I'll take a look.

[mausch]

Upgraded to PerfUtil 0.1.8 in https://github.com/mausch/Fuchu/commit/86c6614f09cc447b141934bdc7d7740235e0a91e