Fix CI on Travis

[merbla]

Currently targeting latest and Preview2

https://travis-ci.org/serilog/serilog-sinks-splunk

This should only target required versions.

Issue with OSX and OpenSSL

==> Downloading https://homebrew.bintray.com/bottles/openssl-1.0.2h_1.yosemite.b
==> Pouring openssl-1.0.2h_1.yosemite.bottle.tar.gz
==> Caveats
A CA file has been bootstrapped using certificates from the system
keychain. To add additional certificates, place .pem files in
  /usr/local/etc/openssl/certs
and run
  /usr/local/opt/openssl/bin/c_rehash
This formula is keg-only, which means it was not symlinked into /usr/local.
Apple has deprecated use of OpenSSL in favor of its own TLS and crypto libraries
Generally there are no consequences of this for you. If you build your
own software and it requires this formula, you'll need to add to your
build variables:
    LDFLAGS:  -L/usr/local/opt/openssl/lib
    CPPFLAGS: -I/usr/local/opt/openssl/include
==> Summary
🍺  /usr/local/Cellar/openssl/1.0.2h_1: 1,691 files, 12.0M
Warning: Refusing to link: openssl
Linking keg-only openssl means you may end up linking against the insecure,
deprecated system OpenSSL while using the headers from Homebrew's openssl.
Instead, pass the full include/library paths to your compiler e.g.:
  -I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib

[merbla]

Still an issue here https://github.com/dotnet/cli/issues/2229