Work around Travis "virtual memory exhausted" error

One particular build was running out of memory. By swapping to debug mode
we reduce the optimisation level which should reduce the amount of memory
required.

[extended tests]
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3601)

.travis.yml

@@ -121,7 +121,7 @@ matrix:
                   sources:
                       - ubuntu-toolchain-r-test
           compiler: gcc-5
-          env: EXTENDED_TEST="yes" CONFIG_OPTS="no-asm enable-ubsan enable-rc5 enable-md2 enable-aria -DPEDANTIC" OPENSSL_TEST_RAND_ORDER=0
+          env: EXTENDED_TEST="yes" CONFIG_OPTS="--debug no-asm enable-ubsan enable-rc5 enable-md2 enable-aria -DPEDANTIC" OPENSSL_TEST_RAND_ORDER=0
         - os: linux
           addons:
               apt: