Bump required Erlang/OTP version to 19.1

Since 538f35d05a, Erlang/OTP 19.1 is
required.

.travis.yml

@@ -1,7 +1,7 @@
 language: erlang
 
 otp_release:
-  - 19.0
+  - 19.1
   - 20.3
   - 21.2
 

configure.ac

@@ -3,7 +3,7 @@
 
 AC_PREREQ(2.53)
 AC_INIT(ejabberd, m4_esyscmd([echo `git describe --tags 2>/dev/null || echo 0.0` | sed 's/-g.*//;s/-/./' | tr -d '\012']), [ejabberd@process-one.net], [ejabberd])
-REQUIRE_ERLANG_MIN="8.0 (Erlang/OTP 19.0)"
+REQUIRE_ERLANG_MIN="8.1 (Erlang/OTP 19.1)"
 REQUIRE_ERLANG_MAX="100.0.0 (No Max)"
 
 AC_CONFIG_MACRO_DIR([m4])