Add crash.log to Packer.gitignore (#3230)

If packer crashes, it write logs to the file "crash.log" in the current directory. This file should not be tracked in version control.

Packer.gitignore

@@ -1,5 +1,8 @@
 # Cache objects
 packer_cache/
 
+# Crash log
+crash.log
+
 # For built boxes
 *.box