Replace uses of boost::filesystem with fs

Step two in abstracting away boost::filesystem.

To repeat this, simply run:
```
git ls-files \*.cpp \*.h | xargs sed -i 's/boost::filesystem/fs/g'
```

src/bitcoin-cli.cpp

@@ -96,7 +96,7 @@ static int AppInitRPC(int argc, char* argv[])
         }
         return EXIT_SUCCESS;
     }
-    if (!boost::filesystem::is_directory(GetDataDir(false))) {
+    if (!fs::is_directory(GetDataDir(false))) {
         fprintf(stderr, "Error: Specified data directory \"%s\" does not exist.\n", GetArg("-datadir", "").c_str());
         return EXIT_FAILURE;
     }