Add clang to the build matrix.
Add clang to the build matrix.

dstufft/cryptography,kimvais/cryptography,Hasimir/cryptography,Hasimir/cryptography,bwhmather/cryptography,Lukasa/cryptography,kimvais/cryptography,Lukasa/cryptography,kimvais/cryptography,Ayrx/cryptography,sholsapp/cryptography,glyph/cryptography,dstufft/cryptography,sholsapp/cryptography,Lukasa/cryptography,Hasimir/cryptography,skeuomorf/cryptography,bwhmather/cryptography,Ayrx/cryptography,dstufft/cryptography,Hasimir/cryptography,dstufft/cryptography,skeuomorf/cryptography,sholsapp/cryptography,Ayrx/cryptography,dstufft/cryptography,Ayrx/cryptography,bwhmather/cryptography,bwhmather/cryptography,skeuomorf/cryptography,sholsapp/cryptography,skeuomorf/cryptography,kimvais/cryptography,glyph/cryptography