Fixed: test_limit_resources (Magick_UT) in Magick.rb fails #126

Error message:
```
Magick_UT#test_limit_resources [/home/linduxed/Documents/rmagick/test/Magick.rb:289]:
Failed assertion, no message given.
```

Magick.rb:289:
```ruby
        assert(cur > 1024)
```

cur:
```
cur = Magick::limit_resource(:file, 500)
```

default file limit: 768 (From PR #91)

From the above:
```
assert(cur > 1024)
=> assert(768 > 1024)
=> assert(false)
=> Failed assertion
```