Update README.md

[setuidroot]

This is a silly PR, but safecurves.cr.yp.to (a site dedicated to cryptography) not using TLS encryption on their webpage seems a little silly to me. Unfortunately http://safecurves.cr.yp.to/ does not automatically redirect to their https TLS encrypted webpage despite their website having a valid Let's Encrypt certificate.

This PR changes it to https://safecurves.cr.yp.to/ so TLS is used when you click on the link here.

[coveralls]

Coverage remained the same at 88.928% when pulling bc3d2ae81cfbcd38d21918ea285925a0a236ab81 on setuidroot:master into e71b2d9359c5fe9437fbf46f1f05096de447de57 on indutny:master.

[fanatid]

Can you also change http:// to https:// in other files (except dist folder)?

$ grep 'http://' -rn .
./dist/elliptic.js:921:  // http://hyperelliptic.org/EFD/g1p/auto-montgom-xz.html#doubling-dbl-1987-m-3
./dist/elliptic.js:946:  // http://hyperelliptic.org/EFD/g1p/auto-montgom-xz.html#diffadd-dadd-1987-m-3
./Gruntfile.js:63:          urls: ['http://127.0.0.1:3000/unittests.html'],
./lib/elliptic/curve/mont.js:83:  // http://hyperelliptic.org/EFD/g1p/auto-montgom-xz.html#doubling-dbl-1987-m-3
./lib/elliptic/curve/mont.js:108:  // http://hyperelliptic.org/EFD/g1p/auto-montgom-xz.html#diffadd-dadd-1987-m-3
./benchmarks/deps/jodid.js:31: * see: http://github.com/jrburke/almond for details
./README.md:1:# Elliptic [![Build Status](https://secure.travis-ci.org/indutny/elliptic.png)](http://travis-ci.org/indutny/elliptic) [![Coverage Status](https://coveralls.io/repos/indutny/elliptic/badge.svg?branch=master&service=github)](https://coveralls.io/github/indutny/elliptic?branch=master) [![Code Climate](https://codeclimate.com/github/indutny/elliptic/badges/gpa.svg)](https://codeclimate.com/github/indutny/elliptic)
./README.md:7:NOTE: Please take a look at http://safecurves.cr.yp.to/ before choosing a curve
./README.md:234:[0]: http://tools.ietf.org/html/rfc6979
./.jshintrc:3:    // See http://jshint.com/docs/ for more details