Increase zmq minimum version, untested fix to gcc-8 support.

(cherry picked from commit 4962edb)

Author: pmienk

generate.xml

@@ -593,7 +593,7 @@
         <option value="--with-thread" />
         <option value="--with-test" />
       </build>
-      <build name="zmq" version="4.2.0" parallel="true" nuget="true" >
+      <build name="zmq" version="4.2.5" parallel="true" nuget="true" >
       </build>
       <build name="secp256k1" github="libbitcoin" repository="secp256k1" branch="version5" parallel="true" nuget="true" >
         <option value="--disable-tests" />
@@ -1193,7 +1193,7 @@
         <option value="--with-thread" />
         <option value="--with-test" />
       </build>
-      <build name="zmq" version="4.2.0" parallel="true" nuget="true" >
+      <build name="zmq" version="4.2.5" parallel="true" nuget="true" >
       </build>
       <build name="secp256k1" github="libbitcoin" repository="secp256k1" branch="version5" parallel="true" nuget="true" >
         <option value="--disable-tests" />
@@ -1699,7 +1699,7 @@
           <nuget compiler="vc141" name="boost_unit_test_framework-vc141" version="1.64.0.0" />
       </dependency>
       <!-- We specify zmq explicitly to require a version with tweetnacl. -->
-      <dependency name="zmq" version="4.2.0">
+      <dependency name="zmq" version="4.2.5">
           <nuget compiler="vc120" name="libzmq_vc120" version="4.2.30" />
           <nuget compiler="vc140" name="libzmq_vc140" version="4.2.30" />
           <nuget compiler="vc141" name="libzmq_vc141" version="4.2.30" />
@@ -1772,7 +1772,7 @@
         <option value="--with-thread" />
         <option value="--with-test" />
       </build>
-      <build name="zmq" version="4.2.0" parallel="true" nuget="true" >
+      <build name="zmq" version="4.2.5" parallel="true" nuget="true" >
       </build>
       <build name="secp256k1" github="libbitcoin" repository="secp256k1" branch="version5" parallel="true" nuget="true" >
         <option value="--disable-tests" />
@@ -1956,7 +1956,7 @@
         <option value="--with-thread" />
         <option value="--with-test" />
       </build>
-      <build name="zmq" version="4.2.0" parallel="true" nuget="true" >
+      <build name="zmq" version="4.2.5" parallel="true" nuget="true" >
       </build>
       <build name="secp256k1" github="libbitcoin" repository="secp256k1" branch="version5" parallel="true" nuget="true" >
         <option value="--disable-tests" />