Update Travis CI files to account for new build dependencies

* Add Fedora 28 and Ubuntu 18.04 Dockerfile
* Add new build dependencies (for the ACRN tools)
* Change default Fedora version to 28

Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>

.travis-dockerfiles/Dockerfile.debian8

@@ -0,0 +1,19 @@
+# Build container based on Debian 8
+FROM debian:8
+
+# Install dependencies.
+RUN apt-get update \
+    && apt-get install -y gcc make vim git \
+                          gnu-efi \
+                          libssl-dev \
+                          libpciaccess-dev \
+                          uuid-dev \
+                          libsystemd-dev \
+                          libevent-dev \
+                          libxml2-dev \
+                          libusb-1.0-0-dev \
+    && apt-get clean
+
+WORKDIR /root/acrn
+
+CMD ["/bin/bash"]