Change travis.sh to check if variables are equal to yes instead of

not-empty.
Before this, setting `VALGRIND=wat` was considered as true, and to make it
evaluate as false you had to unset the variable `VALGRIND=` but not it
checks if `VALGRIND=yes` and if it's not `yes` then it's evaluated to
false

contrib/travis.sh

@@ -23,7 +23,7 @@ if [ -n "$BUILD" ]
 then
     make -j2 "$BUILD"
 fi
-if [ -n "$VALGRIND" ]
+if [ "$VALGRIND" = "yes" ]
 then
     make -j2
     # the `--error-exitcode` is required to make the test fail if valgrind found errors, otherwise it'll return 0 (http://valgrind.org/docs/manual/manual-core.html)
@@ -32,7 +32,7 @@ then
 fi
 if [ -n "$BENCH" ]
 then
-    if [ -n "$VALGRIND" ]
+    if [ "$VALGRIND" = "yes" ]
     then
         # Using the local `libtool` because on macOS the system's libtool has nothing to do with GNU libtool
         EXEC='./libtool --mode=execute valgrind --error-exitcode=42'
@@ -56,7 +56,7 @@ then
         $EXEC ./bench_ecdh >> bench.log 2>&1
     fi
 fi
-if [ -n "$CTIMETEST" ]
+if [ "$CTIMETEST" = "yes" ]
 then
     ./libtool --mode=execute valgrind --error-exitcode=42 ./valgrind_ctime_test > valgrind_ctime_test.log 2>&1
 fi