Fix getting long serial numbers for openssl < 1.1

Add a CMake option to disable shared library

From https://www.ietf.org/rfc/rfc5280.txt

> As noted in Section 4.1.2.2, serial numbers can be expected to
> contain long integers.  Certificate users MUST be able to handle
> serialNumber values up to 20 octets in length.  Conforming CAs MUST
> NOT use serialNumber values longer than 20 octets.

Without this, nghttpx will fatal.

    jbraeg$ openssl x509 -in ~/test_certs/client.crt -serial -noout
    serial=E0CFDFC7CEA10DF8AAF715C37FAEB410

    jbraeg$ curl -k --key ~/test_certs/client.key --cert ~/test_certs/client.crt https://192.168.98.100:3000/; echo
    curl: (56) Unexpected EOF

    ...
    Assertion failed: n == b.size() (shrpx_tls.cc: get_x509_serial: 2051)
    2019-01-03T20:25:21.289Z 1 1 f84316ae NOTICE (shrpx_log.cc:895) Worker process: [9] exited abnormally with status 0x06; exit status 0; signal Aborted(6)
    2019-01-03T20:25:21.290Z 1 1 f84316ae NOTICE (shrpx.cc:4311) Shutdown momentarily

Signed-off-by: Brendan Heinonen <brendan@heinonen.co>

Travis: Update to Xenial

clang-7 is enable by default

Xenial is now available with Travis CI
https://blog.travis-ci.com/2018-11-08-xenial-release

nghttpx allows a pattern with trailing slash to match a request path
without it.  Previously, under certain pattern registration, this does
not work.

h2load: Handle HTTP/1 non-final response

cmake: Fix libevent version detection

Use __has_declspec_attribute for shared builds