Fix luarocks lint

Now it does not crash on rockspecs with malformed version. 

Testcase: 

```$ echo 'version = "bad"' > testcase.rockspec```

Before: 

```
$ luarocks lint testcase.rockspec

Error: LuaRocks 2.2.0 bug (please report at luarocks-developers@lists.sourceforge.net).
/usr/local/share/lua/5.2/luarocks/type_check.lua:219: attempt to concatenate global 'expected' (a nil value)
stack traceback:
	/usr/local/share/lua/5.2/luarocks/type_check.lua:219: in function 'type_check_item'
	/usr/local/share/lua/5.2/luarocks/type_check.lua:276: in function </usr/local/share/lua/5.2/luarocks/type_check.lua:263>
	(...tail calls...)
	/usr/local/share/lua/5.2/luarocks/fetch.lua:198: in function 'load_local_rockspec'
	/usr/local/share/lua/5.2/luarocks/lint.lua:37: in function </usr/local/share/lua/5.2/luarocks/lint.lua:21>
	(...tail calls...)
	[C]: in function 'xpcall'
	/usr/local/share/lua/5.2/luarocks/command_line.lua:208: in function 'run_command'
	/usr/local/bin/luarocks:33: in main chunk
	[C]: in ?
```

After: 

```
$ luarocks lint testcase.rockspec

Error: Failed loading rockspec: /home/mpeterv/test/testcase.rockspec: Type mismatch on field version: invalid value bad does not match '[%w.]+-[%d]+'
```