Remove needless dependencies in Gemfile

Specify the location of extconf.rb in gemspec

This commit fixes the following errors:

```
linking shared-object ruby_gumath/ruby_gumath.bundle
duplicate symbol _cGumath_GufuncObject in:
    util.o
    gufunc_object.o
duplicate symbol _cGumath in:
    util.o
    gufunc_object.o
duplicate symbol _GufuncObject_type in:
    util.o
    gufunc_object.o
duplicate symbol _cGumath_GufuncObject in:
    util.o
    functions.o
duplicate symbol _cGumath in:
    util.o
    functions.o
duplicate symbol _GufuncObject_type in:
    util.o
    functions.o
duplicate symbol _cGumath_GufuncObject in:
    util.o
    ruby_gumath.o
duplicate symbol _cGumath in:
    util.o
    ruby_gumath.o
duplicate symbol _GufuncObject_type in:
    util.o
    ruby_gumath.o
duplicate symbol _cGumath_GufuncObject in:
    util.o
    examples.o
duplicate symbol _cGumath in:
    util.o
    examples.o
duplicate symbol _GufuncObject_type in:
    util.o
    examples.o
ld: 12 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)```
```

Fix duplicate symbols error