Do not error if there are no 'resolved' flag files

Plugins with no dependencies (like the `ansicolor` plugin) will not have created a `resolved` flag. 

`rm` will throw an error if the file does not exist. Set a `-f` flag to stop this.
diff --git a/install-plugins.sh b/install-plugins.sh
index 630b8cd..e1150fc 100755
--- a/install-plugins.sh
+++ b/install-plugins.sh
@@ -81,4 +81,4 @@
 done
 
 # cleanup 'resolved' flag files
-rm *.resolved
+rm -f *.resolved