Add freetype to brew install pre-reqs

[highb]

When running pip install, it error-ed out because I didn't have FreeType installed. This was the specific message:

    clang -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -DFREETYPE_BUILD_TYPE=system -DPY_ARRAY_UNIQUE_SYMBOL=MPL_matplotlib_ft2font_ARRAY_API -DNPY_NO_DEPRECATED_API=NPY_1_7_API_VERSION -D__STDC_FORMAT_MACROS=1 -Iextern/agg24-svn/include -I/Users/brandonhigh/.asdf/installs/python/3.9.0/lib/python3.9/site-packages/numpy/core/include -I/Users/brandonhigh/.asdf/installs/python/3.9.0/include/python3.9 -c src/checkdep_freetype2.c -o build/temp.macosx-10.15-x86_64-3.9/src/checkdep_freetype2.o
    src/checkdep_freetype2.c:3:6: error: "FreeType version 2.3 or higher is required. You may set the MPLLOCALFREETYPE environment variable to 1 to let Matplotlib download it."
        #error "FreeType version 2.3 or higher is required. \
         ^
    src/checkdep_freetype2.c:9:10: fatal error: 'ft2build.h' file not found
    #include <ft2build.h>
             ^~~~~~~~~~~~
    2 errors generated.
    error: command '/usr/bin/clang' failed with exit code 1

After brew install freetype, I was able to pip install with no issues.

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[w0rmr1d3r]

Could this PR be somehow related to this issue -> https://github.com/duo-labs/cloudmapper/issues/803 ?

[highb]

@w0rmr1d3r I believe so.

[0xdabbad00]

LGTM, sorry for the delay.