Summary:
CMake find_package modules are case sensitive. Since the script is called
"FindLZ4.cmake" the corresponding call to `find_package` must match LZ4
and not "lz4".

For an example of the problem, see the [Travis Log](https://travis-ci.org/facebook/folly/jobs/345661072). Namely:

```
CMake Warning at CMake/folly-deps.cmake:101 (find_package):
  No "Findlz4.cmake" found in CMAKE_MODULE_PATH.
Call Stack (most recent call first):
  CMakeLists.txt:65 (include)
CMake Warning (dev) at CMake/folly-deps.cmake:101 (find_package):
  Findlz4.cmake must either be part of this project itself, in this case
  adjust CMAKE_MODULE_PATH so that it points to the correct location inside
  its source tree.
```

This was not caught on my local machine as I use case-insensitive options in my terminal; in addition, this is not a "hard error" for CMake, so the builds happily continued successfully when 7dc5a4ed landed.
Closes https://github.com/facebook/folly/pull/782

Reviewed By: simpkins

Differential Revision: D7079535

Pulled By: yfeldblum

fbshipit-source-id: dd51d8c6ffa804be72c2d9cd264890280dcf5ab9