Added null-argument checks for functions in the Choice module, where a null Choice`2 value could potentially be passed in (e.g., if consuming from C#).
Fixed a typo in the solution file that caused issues when trying to build profile259 builds in VS.
Disable profile78 builds in TravisCI for now, they're failing due to the profile78 version of FSharp.Core not being found on the Mono installations on TravisCI machines.