<apiAccessLevel>Unrestricted</apiAccessLevel>
<namespacePrefix>your_ns</namespacePrefix>

<target name="push" description="Deploys all package components">
    <sf:deploy username="${sf.username}" password="${sf.password}" serverurl="https://login.salesforce.com" deployRoot="src"/>
</target>

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <fullName>DevMuza</fullName>
    <apiAccessLevel>Unrestricted</apiAccessLevel>
    <description>DevMuza Package to test CircleCI.</description>
    <namespacePrefix>dev_muza</namespacePrefix>
    <types>
        <members>ContactCounter_Ext</members>
        <members>ContactCounter_Ext_UT</members>
        <name>ApexClass</name>
    </types>
    <types>
        <members>CountContacts</members>
        <name>ApexPage</name>
    </types>
    <types>
        <members>Account.Number_of_Contacts__c</members>
        <name>CustomField</name>
    </types>
    <version>40.0</version>
</Package>

...
  <target name="updatePackage">
    <sf:deploy username="${sf.username}" password="${sf.password}" sessionId="${sf.sessionId}" serverurl="${sf.serverurl}" maxPoll="${sf.maxPoll}" deployRoot="." singlePackage="true" />
  </target>
...