Fix initial memory limit test

I took the "correct" value right from the RSpec output on Travis CI:

    test_limit_resources(Magick_UT)
    [/home/travis/build/gemhome/rmagick/test/Magick.rb:268]:
    <782635008> expected but was <3221225472>.

In fact, this value is machine and OS-dependent.
ImageMagick 6.8.9 (r15243) has the following definition:

magick/resource.c:111:

    MagickULLConstant(1536)*1024*1024, /* memory limit */

magick/magick-type.h:36:

    #  define MagickULLConstant(c)  (MagickSizeType) (c ## ULL)

magick/magick-type.h:151:

    typedef unsigned long long MagickSizeType;

When building on Windows, other definitions take place.
So, I think the initial value should not be in the test at all.
But let it be there for now, to test that the Ruby library correctly
gets the value from the C extension that correctly gets it from
ImageMagick that is running on Travis machines.
Do not test machine and OS-specific integers
Test if default limits are somewhat reasonable
Remove obsolete comment
Merge pull request #91 from vassilevsky/limits-testing

Do not test machine and OS-specific integers
Fixed: build error on Windows x64 (with ImageMagick 6.8.0-10 or ImageMagick 6.8.7-7)
Merge pull request #94 from u338steven/patch-1

Fixed: build error on Windows Ruby x64 (with ImageMagick 6.8.0-10 or Ima...
Test on multiple versions of Ruby.
Fix pixel hash test
Process the return value of Pixel_hash() with UINT2NUM
Merge pull request #95 from gemhome/pixel-hash-test-fix

Fix pixel hash test
Fixed: build error with ImageMagick 6.8.8
LABColorspace -> LabColorspace
MaxRGB -> QuantumRange
QuantumDepth -> MAGICKCORE_QUANTUM_DEPTH
Move @@tmpnam test to a separate file for isolation
Fixed: test_convolve(Image2_UT) in Image2.rb fails #72
Fixed: test_export_pixels_to_str(Image2_UT) in Image2.rb fails
Fixed: test_resample(Image3_UT) in Image3.rb fails #74
Merge pull request #100 from u338steven/patch-4

Fixed: test_resample(Image3_UT) in Image3.rb fails #74