Remove reject column from UCBadmit

[JohannesNE]

The examples using UCBadmit gives an error with Stan 2.21.0. This is caused by the reject column in UCBadmit, as reject is apparently a reserved keyword in Stan.

Semantic error in '/tmp/RtmpLZPZxd/model-43b00def6d21.stan', line 2, column 8 to column 14:
   -------------------------------------------------
     1:  data{
     2:      int reject[12];
                 ^
     3:      int applications[12];
     4:      int admit[12];
   -------------------------------------------------

Identifier 'reject' clashes with reserved keyword.

make: *** [make/program:50: /tmp/RtmpLZPZxd/model-43b00def6d21.hpp] Error 1

Error: An error occured during compilation! See the message above for more information.

[rmcelreath]

Which example in the book causes this? I thought this is fixed in the second edition already, so no change is required.

[JohannesNE]

Not sure any examples from the book causes a problem. I only noticed the problem trying to run the examples from the README https://github.com/rmcelreath/rethinking/blob/master/README.md

[JohannesNE]

Oh, and feel free just to close this pull request if you prefer fixing it in an other way.