.bash_profile: Make sure only files get sourced

Closes mathiasbynens#250.

.bash_profile

@@ -5,7 +5,7 @@ export PATH="$HOME/bin:$PATH"
 # * ~/.path can be used to extend `$PATH`.
 # * ~/.extra can be used for other settings you don’t want to commit.
 for file in ~/.{path,bash_prompt,exports,aliases,functions,extra}; do
-	[ -r "$file" ] && source "$file"
+	[ -r "$file" ] && [ -f "$file" ] && source "$file"
 done
 unset file