upgrade pyelliptic from 1.5.2 to 1.5.3

Because of this issue https://github.com/yann2192/pyelliptic/issues/17 I'd recommend upgrading pyelliptic to 1.5.3.  Note, however, that 1.5.2 and 1.5.3 ciphertexts are incompatible so old clients would not work with new ones.