Fixing rippled build errors

If you get an error that looks like this:

...
 * [new tag]             v6.8.1               -> v6.8.1
error: Your local changes to the following files would be overwritten by checkout:
	thirdparty.inc
	util/build_version.cc.in
Please commit your changes or stash them before you switch branches.
Aborting
CMake Error at /Users/user/ripple/rippled/.nih_c/unix_makefiles/AppleClang_10.0.0.10001145/Debug/tmp/rocksdb-gitupdate.cmake:147 (message):
  Failed to checkout tag: 'v6.7.3'


make[2]: *** [../.nih_c/unix_makefiles/AppleClang_10.0.0.10001145/Debug/src/rocksdb-stamp/rocksdb-update] Error 1
make[1]: *** [CMakeFiles/rocksdb.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
...
[ 21%] Built target grpc_src
make: *** [all] Error 2

Try deleting the ../.nih_c directory, which is an object cache for external projects that rippled depends on.

https://github.com/ripple/rippled/blob/master/Builds/CMake/RippledNIH.cmake