"snap" images are updated frequently, while regular images are updated quarterly.
so, mixing "snap" and regular images lead to package naming mismatch, which will occur every
quarter. we cannot use 11.3 release image, it is broken, so we have to use 11.3 "snap" image.
Thus let us use all "snap" images. 13-snap is first introduced with this commit.
cirrus ci builds are now limited to branches master, next
travis-ci images are upgraded to ubuntu bionic
cygwin builds are temporarily disabled on travis-ci
(maybe someone will figure out how to fix them, here's link
https://travis-ci.community/t/cygwin-issue-cygheap-base-mismatch-detected/5359/2 )
openssl upgraded to 1.0.2t, 1.1.0l, 1.1.1d
libressl are upgraded (2.9.2, 2.8.3, 2.7.5) --> (3.0.2, 2.9.2, 2.8.3)
