Add set -e -x to scripts for travis

`-e`: exit immediately if a simple command exits with a non-zero status
`-x`: print a trace of simple commands and their arguments after they
      are expanded and before they are executed

.ci/travis-after-success.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+set -e -x
+
 if [ "${BUILD}" == "tests" ]; then
     codecov
 fi

.ci/travis-before-install.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+set -e -x
+
 if [ "${BUILD}" != "flake8" ]; then
     apt-get update -qq
     apt-get -y install flex bison libgmp-dev libmpc-dev python-dev libssl-dev

.ci/travis-install.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+set -e -x
+
 pip install --upgrade pip
 
 # will not be needed for flake8

.ci/travis-script.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+set -e -x
+
 if [ "${BUILD}" == "tests" ]; then
     pytest -v --cov=honeybadgerbft
 elif [ "${BUILD}" == "flake8" ]; then