Issue #67 : fix a bug about dofile

[kohkimakimoto]

Fixes #67 .

[coveralls]

Coverage decreased (-0.01%) to 92.379% when pulling 98b2068 on kohkimakimoto:fix-bug-dofile into b5ca24a on yuin:master.

[kohkimakimoto]

To fix the issue, I updated code in the two places.
I tested this code by using glua. I got the below result.

a.lua

dofile "b.lua"

b.lua

error("error!")

$ glua a.lus
b.lua:1: error!
stack traceback:
    [G]: in function 'error'
    b.lua:1: in function <b.lua:0>
    [G]: in function 'dofile'
    a.lua:1: in main chunk
    [G]: ?

Please merge it if you want.

[yuin]

Thanks for your PR.

But it seems just dofile bug. Lua reference manual says "that is, dofile does not run in protected mode" .
So I've changed PCall to Call in dofile .