Modify .travis.yml to explictly pass no in env vars instead of setting to nothing

Author: elichai

.travis.yml

@@ -31,9 +31,9 @@ env:
     - BIGNUM=no
     - BIGNUM=no       ENDOMORPHISM=yes RECOVERY=yes EXPERIMENTAL=yes
     - BIGNUM=no       STATICPRECOMPUTATION=no
-    - BUILD=distcheck CTIMETEST= BENCH=
+    - BUILD=distcheck CTIMETEST=no BENCH=no
     - CPPFLAGS=-DDETERMINISTIC
-    - CFLAGS=-O0 CTIMETEST=
+    - CFLAGS=-O0 CTIMETEST=no
     - ECMULTGENPRECISION=2
     - ECMULTGENPRECISION=8
     - VALGRIND=yes ENDOMORPHISM=yes BIGNUM=no ASM=x86_64 EXPERIMENTAL=yes ECDH=yes  RECOVERY=yes EXTRAFLAGS="--disable-openssl-tests" CPPFLAGS=-DVALGRIND BUILD=