Update .travis.yml

Author: hazzik

.travis.yml

@@ -1,4 +1,4 @@
-dist: bionic
+dist: xenial
 language: csharp
 mono: none
 dotnet: 2.1.300
@@ -27,7 +27,6 @@ before_install:
     if [[ "$DB" == "Firebird" ]]
     then
       sudo apt-get install -y libicu-dev libtommath-dev
-      sudo ln -s /usr/lib/x86_64-linux-gnu/libtommath.so.1 /usr/lib/x86_64-linux-gnu/libtommath.so.0
       wget -q https://github.com/FirebirdSQL/firebird/releases/download/R3_0_5/Firebird-3.0.5.33220-0.amd64.tar.gz
       tar xzvf Firebird-3.0.5.33220-0.amd64.tar.gz
       pushd Firebird-3.0.5.33220-0.amd64