Changed to CC BY 4.0 license

After a little bit more research, I decided to switch to the CC-BY 4.0 license for the SRD. This seemed to be the right thing. Even though I would love it if people make materials using it to make their materials share alike, I don't think it should be required. (I did leave Be You Bold sharalike). For an SRD, I think it makes sense that everyone should be able to make the project they want and I had the thought that making it sharealike might make some individuals hesitant to use it.